Phosgene having low content of carbon tetrachloride

ABSTRACT

A process for producing phosgene having low content of carbon tetrachloride is disclosed. The process entails reacting carbon monoxide with chlorine in the presence of elemental carbon, in a reactor. Critically the temperature of the gas stream emerging from the reactor is restricted to 30 to 80° C. and its pressure, measured directly downstream from said reactor is restricted to 120 to 400 kPa abs . The phosgene thus produced has a content of carbon tetrachloride that is less than 150 ppm.

This invention relates to phosgene which is low in carbon tetrachloride,to a process for the production thereof, and to the use thereof forproducing polycarbonates and polyester carbonates.

Phosgene is an important starting material for producing intermediatesand final products in many branches of chemistry, particularly for theproduction of isocyanates, polycarbonate plastics and esters ofchloroformic acid. Pure phosgene is a colourless, toxic gas which issusceptible to hydrolysis. Under normal pressure it boils at −127.8° C.and condenses at 7.56° C. to form a colourless liquid.

It is known that phosgene can be produced by the catalytic combinationof carbon monoxide (CO) and elemental chlorine (Cl₂):

Activated carbon is mostly used as the catalyst. The reaction isexothermic and the conversion is almost complete. The reaction isusually conducted in tubular reactors which are made from standard steelor stainless steel and which are packed with catalyst. These tubes aregenerally cooled externally in order to dissipate the heat of reaction.Depending on the processing conditions, the phosgene which is producedby this process contains varying amounts of carbon tetrachloride (CCl₄)as an impurity. This is formed by unwanted secondary reactions such asdisproportionation reactions, excessive chlorination of the carbonmonoxide, chlorination of the carbon catalyst and chlorination ofmethane (CH₄). The degree of contamination by carbon tetrachloride ofphosgene which is produced by commonly-used processes usually fallswithin the range from 250 to 2000 ppm.

The formation of carbon tetrachloride as a by-product during theproduction of phosgene is problematical from many aspects. Thus, due tothe environmental harmfulness of chlorinated hydrocarbons, costlyprecautions have to be taken to ensure that the carbon tetrachlorideformed in the reactions is not discharged into the environment.Moreover, even slight contents of carbon tetrachloride have adisadvantageous effect on some products which are manufactured fromphosgene, particularly plastics. For example, high-purity phosgene witha carbon tetrachloride content of 250 ppm at most is required for theproduction of polycarbonate plastics, in order to prevent discolorationof the final product, which is normally colourless and transparent.

JP 88-156040 describes a process for producing phosgene which is low incarbon tetrachloride by the reaction of carbon monoxide with chlorineover activated carbon at 350° C., wherein the activated carbon catalystis purified before reaction by treatment with acid to remove impuritiessuch as transition metals, boron, aluminium and silicon. The phosgenewhich is produced by this process has a carbon tetrachloride content of150 ppm. A disadvantage of this process, however, is the costlypretreatment of the activated carbon catalyst.

JP 80-014044 proposes a reactor for the conversion of chlorine andcarbon monoxide into phosgene, in which the activated carbon catalyst ispositioned at the reactor walls with the aid of a metal mesh. Thisarrangement prevents overheating of the catalyst bed. The phosgene whichis produced in a reactor such as this at temperatures of 200 to 250° C.and at a throughput of 600 to 1200 I/h has a carbon tetrachloridecontent of 250 to 400 ppm. A disadvantage of this process, however, isthat repacking the reactor with activated carbon when the activity ofthe latter decreases is more expensive than is the operation ofconventional tubular reactors.

The underlying object of the present invention is to provide phosgenewith an even lower carbon tetrachloride content.

This object is achieved by a grade of phosgene which contains less thanthe 150 ppm of carbon tetrachloride. The present invention furtherrelates to a process for producing a grade of phosgene such as this bythe reaction of carbon monoxide with chlorine in the presence ofelemental carbon at a temperature of 30 to 80° C. and at a pressure of120 to 400 kpa_(abs) as measured directly downstream of the phosgenegenerator.

It is advantageously possible to conduct the process according to theinvention in customary tubular reactors made of standard steel orstainless steel, the tubes of which are packed with the carbon catalyst.The tubular reactor can be operated continuously or batch-wise.

Carbon tetrachloride and chlorine are introduced into the reactor inapproximately equal parts, preferably at room temperature. To ensurethat all the chlorine is reacted, a slight excess of carbon monoxide canbe used. Before they enter the reactor, the two reactants are preferablymixed in a suitable mixing device, in a static mixer for example.

Another advantage of the process according to the invention is that nospecial preparation of the catalyst is necessary.

The temperature of the gas stream emerging from the reactor should notexceed 70 to 80° C., as measured directly downstream of phosgenegenerator. The temperature of the gas stream emerging from the reactorin the process according to the invention is preferably 40 to 70° C. Thegas stream is prevented from exceeding these temperatures with the aidof appropriate cooling devices which dissipate the heat of reactionwhich is released during the reaction and which prevent any overheatingof the catalyst.

Activated carbon is preferably used as the carbon catalyst. Granularactivated carbon with a grain diameter of 3 to 4 mm is preferably usedas the catalyst. The pore surface area of the activated carbon ispreferably about 1000 m²/g. The apparent density of the activated carbonused is preferably about 450 g/l.

The pressure which is measured directly downstream of the phosgenereactor is preferably 300 kPa_(abs), at most. It is thereby ensured thatphosgene cannot condense in the reactor.

The phosgene which is produced by the process according to the inventionand which emerges from the top of the reactor is preferably condensed attemperatures of −10 to −20° C. Due to its low carbon tetrachloridecontent of less than 150 ppm, it can be used directly, without furtherpurification, for producing polymers comprising a carbonate bond,particularly polycarbonates or polyester carbonates.

The chlorine which is used in the reaction can be produced by customaryindustrial processes such as chlor-alkali electrolysis or hydrogenchloride electrolysis, and should be as pure as possible. Chlorine witha degree of purity higher than 98% is particularly suitable. Liquidchlorine from a storage vessel is preferably used. This is evaporated ina heated gasifier and is subsequently freed from any entrained liquidchlorine in a re-evaporator.

The carbon monoxide which is used in the reaction can be produced bycustomary methods, for example from natural gas/naphtha in a synthesisgas installation or by blowing coke with oxygen. It has proved to beparticularly advantageous if the carbon monoxide has a methane contentof less than 50 ppm.

The invention is explained in more detail below by means of examples.

EXAMPLE 1

500 Nm³/h carbon monoxide with a purity of 97.5% and a methane contentof 50 ppm as determined by on-line IR measurement (Method2301-0207901-91D), and 480 Nm³/h chlorine with a purity of 99.9% weremixed at room temperature and were fed to a phosgene generator packedwith activated carbon (grain diameter about 4 mm, pore surface areaabout 1000 m²/g, apparent density about 450 g/l). The generator wascooled so that the temperature of the phosgene formed, as measureddirectly after the phosgene left the generator, was 55° C. The pressureas measured directly after the phosgene left the generator was 80kPa_(gauge) (180 kPa_(abs)). The phosgene produced had a carbontetrachloride content of 100 ppm.

EXAMPLE 2

500 Nm³/h carbon monoxide with a purity of 98% and a methane content of50 ppm as determined by on-line IR measurement (Method2301-0207901-91D), and 480 Nm³/h chlorine were mixed at room temperatureand were fed to a phosgene generator packed with activated carbon (graindiameter about 4 mm, pore surface area about 1000 m²/g, apparent densityabout 450 g/l). The generator was cooled so that the temperature of thephosgene formed, as measured directly after the phosgene left thegenerator, was 40° C. The pressure as measured directly after thephosgene left the generator was 40 kPa_(gauge) (140 kPa_(abs)). Thephosgene produced had a carbon tetrachloride content of 80 ppm.

COMPARATIVE EXAMPLE

500 Nm³/h carbon monoxide with a purity of 98% and a methane content of10 ppm were mixed at room temperature with 485 Nm³/h chlorine and werefed to a phosgene generator as in Examples 1 and 2. The generator wascooled so that the temperature of the phosgene formed, as measureddirectly after it left the generator, was 85° C. The pressure, which waslikewise measured directly after the phosgene left the generator, was600 kPa_(gauge) (700 kPa_(abs)). The phosgene produced had a carbontetrachloride content of 290 ppm.

1. A process for producing phosgene having carbon tetrachloride contentof less than 150 ppm by the reaction of carbon monoxide with chlorine inthe presence of elemental carbon in a reactor, characterized in thecomplete reaction of the chlorine ensured by using carbon monoxide inslight excess and in that the gas stream emerging from the reactor is ata temperature of 30 to 80° C. and is under a pressure of 120 to 400kPa_(abs) as measured directly downstream of the phosgene generator. 2.A process according to claim 1, characterized in that the gas streamemerging from the reactor is at a temperature of 40 to 70° C.
 3. Aprocess according to claim 1 characterized in that the gas streamemerging from the reactor is under a pressure of 300 kPa_(abs) at most.4. A process according to claim 1 characterized in that the methanecontent of the carbon monoxide is 50 ppm at most.
 5. In the process forproducing phosgene by reacting a slight excess of carbon monoxide withchlorine the improvement comprising carrying out the reaction in thepresence of elemental carbon, restricting the gas stream emerging fromsaid reactor to a temperature of 30 to 80° C. to a pressure of 120 to400 kPa_(abs) as measured directly downstream from said reactor, saidphosgene characterized in having a content of carbon tetrachloride thatis less than 150 ppm.